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Biomechanics
Orthodontics
IN
PRINCIPLES AND PRACTICE
Ram S. Nanda,
BDS, DDS, MS, PhD
Professor Emeritus
Department of Orthodontics
College of Dentistry
University of Oklahoma
Oklahoma City, Oklahoma
Yahya S. Tosun,
DDS, PhD
Private Practice
Dubai, United Arab Emirates
Former Professor
Department of Orthodontics
University of Aegea
İzmir, Turkey
Quintessence Publishing Co, Inc
Chicago, Berlin, Tokyo, London, Paris, Milan, Barcelona,
Istanbul, Moscow, New Delhi, Prague, São Paulo, and Warsaw
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Contents
Preface vii
1
2
3
4
5
6
7
8
9
Physical Principles
1
Application of Orthodontic Force
17
Analysis of Two-Tooth Mechanics
55
Frictional and Frictionless Systems
71
Anchorage Control 83
Correction of Vertical Discrepancies
99
Correction of Transverse Discrepancies
125
Correction of Anteroposterior Discrepancies
Space Closure
Glossary 154
Index 156
145
133
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Preface
Once comprehensive diagnosis and treatment planning
have set the stage for initiating treatment procedures,
appliance design and systems have to be developed to
achieve treatment goals. Correct application of the principles of biomechanics assists in the selection of efficient
and expedient appliance systems.
Over the last three decades, there has been an explosion in the development of technology related to orthodontics. New materials and designs for brackets,
bonding, and wires have combined to create a nearly
infinite number of possibilities in orthodontic appliance
design. As these new materials are brought together in
the configuration of orthodontic appliances, it is necessary to understand and apply the principles of biomechanics for a successful and efficient treatment outcome.
Lack of proper understanding may not only set up inefficient force systems but also cause collateral damage to the tissues. The path to successful treatment is
through good knowledge of biomechanics.
This book is written with the purpose of introducing
a student of orthodontics to the evolving technology,
material properties, and mechanical principles involved
in designing orthodontic appliances.
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3 | Analysis of Two-Tooth Mechanics
Fig 3-20 Four examples of mechanics used to
extrude a canine. (a) An open coil spring between the lateral incisor and premolar on 0.016inch stainless steel wire maintains the space
while preventing the adjacent teeth from tipping.
(b) A cantilever with a V-bend can be used to
move the canine down. The cantilever should be
attached to the canine with a ligature at only one
point to avoid unwanted moment. (c) Reciprocal
anchorage to level maxillary and mandibular canines with an up-and-down elastic. (d) An auxiliary 0.014 or 0.016 NiTi wire can be used along
with a rectangular SS main archwire to bring the
high canine down.
a
b
c
d
Fig 3-21 Laceback prevents the crown from tipping and helps correct the inclination of an upright canine.
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When the wire is engaged in the incisor brackets, three
possible effects can be observed.
First is the rowboat effect, which is caused by a counterclockwise moment on the canine that strains the anchorage.10–12 This moment tends to push the crown forward, resulting in incisor protrusion, which can be
prevented only by means of a laceback.
In Class II, division 1 extraction cases, the rowboat
effect is an undesirable side effect because of the round
tripping or jiggling effect,13–15 which may occur during
retraction of anterior teeth and result in root resorption.
Laceback can prevent the canine crown from tipping
forward. In Class II, division 2 nonextraction cases, incisor protrusion may be desirable; therefore, use of a
straight wire will help induce anterior protrusion as well
as quick alignment.
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Conclusion
a
c
b
Fig 3-22 (a) To avoid extrusion, if a straight wire passing through a vertically positioned canine bracket passes below the incisors, one should not
place the wire in the incisor brackets. (b and c) In this case, instead of using nickel titanium (NiTi) wires, thin stainless steel wires with step-up bends
bypassing the incisor brackets may be used. This is a desirable approach for treating anterior open bites needing correction by incisor extrusion.
a
b
Fig 3-23 (a) The bowing effect may cause bite deepening because of the canine position. (b) If a bendable wire is used, a step-up can be bent
to bypass the incisors gingivally. If the stepped arch is engaged in the anterior brackets, it will cause the canine crown to tip distally because of the
clockwise moment.
The second effect is deepening of the bite (bowing effect; Fig 3-23a). Deepening of the bite during treatment
is usually not a desired effect unless it is indicated in an
anterior open bite that needs to be corrected by maxillary incisor extrusion. To diagnose or predetermine this
effect, place the archwire in the canine bracket slot before ligating it. If the anterior part of the wire runs below
the incisor brackets, it should not be tied to the brackets to avoid incisor extrusion. If a bendable wire is used,
a step-up can be bent to bypass the incisors gingivally.
Another method is to place a continuous intrusion arch
along with the straight wire. The extrusive effect of the
straight wire would therefore be compensated for by
the intrusion arch.
If the stepped, bypass archwire is not left passive and
is engaged in the incisors to intrude them or prevent
them from extruding, it will cause the canine crown to
tip more distally owing to the clockwise moment occurring on its bracket (Fig 3-23b).
In the explanations above, the main reasons for the
adverse effects are the positions or axial inclinations of
the teeth or the brackets. If the problem is caused by
the axial inclinations of the canines, it is important to
upright them with a laceback before inserting a continuous wire.
Conclusion
In the analysis of the relationship between two teeth,
the slot sizes and bracket widths are assumed to be
equal in all the examples given here. Naturally, as the
slot sizes and widths change, the magnitudes of the balancing forces and the moments also change. In clinical
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6 | Correction of Vertical Discrepancies
Tongue
a
Fig 6-35 In open bite cases, control of vertical movement of the molars can be achieved effectively with a high-pull headgear–transpalatal
arch combination. If the transpalatal arch crosses the palate 2 to 3
mm away from the mucosa, the molars will be intruded by vertical
tongue forces during swallowing.
b
Fig 6-36 (a and b) In open bite cases, erupting second molars can
be controlled using a 0.016 0.022–inch SS segmented arch that
passes through the auxiliary tube of the first molar.28
Fig 6-37 Using reverse-curved archwires to close an anterior open
bite. The strong anterior box elastics prevent the premolars from erupting, while the molars intrude and tip back and the incisors extrude.
These mechanics work quite effectively in a very short time, but they
are heavily dependent on patient cooperation. Elastics must be worn
all day, otherwise the bite may open with quick extrusion of the premolars.
trol their eruption before they reach the occlusal plane.
For this purpose, a 0.016 0.022–inch SS segmented
arch can be used as an occlusal stop28 (Fig 6-36).
Arches with reverse curve of Spee
Anterior open bite can be closed with a combination of
a reverse-curved archwire and anterior box elastics31,32
(Fig 6-37). The archwire tends to extrude the maxillary
and mandibular premolars, opening the bite, while
strong anterior box elastics prevent eruption of the premolars and extrude the anteriors. Because the premolars cannot erupt, the molars intrude and tip back with
reciprocal forces. These mechanics effectively close the
bite in 1 or 2 months, but they are heavily dependent on
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patient cooperation. If the patient fails to wear the elastics, the premolars will extrude and cause the bite to
open more. Even though this approach is very effective
in closing the bite, the elastics should not be worn
longer than 2 months because of the possibility of gingival recession and a gummy smile from overeruption
of the incisors.
Molar intrusion with microimplant anchorage
Molar intrusion may be required to control the vertical
discrepancy in skeletal open bite. However, using conventional techniques, this movement is one of the most
challenging procedures in orthodontics—depending on
strong anchorage—but intraoral anchorage is usually not
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Treatment of High-Angle Cases and Correction of Open Bite
Fig 6-38 Molar intrusion using two TADs.
Fig 6-39 Molar intrusion with one TAD and a transpalatal arch.
Fig 6-40 Molar protraction in combination with intrusion may result
in counterclockwise rotation of the mandible, thus helping to correct
the skeletal open bite by reducing lower facial height.
enough without extrusion of adjacent teeth. High-pull
headgear with long arms in conjunction with a transpalatal arch is usually needed to achieve effective intrusion of posterior teeth (see Figs 5-7 and 6-35). Microimplant anchorage also is a very effective way to
intrude molars.
There are two basic methods to intrude molars with
microimplant anchorage:
• Two TADs can be inserted, both buccally and palatally,
and elastic traction applied to the hooks (Fig 6-38). If
two or more posterior teeth need intrusion, the force
can be applied to the archwire.
• One TAD can be inserted buccally, while a transpalatal arch controls buccolingual tipping of the
molar (Fig 6-39). Tongue forces during swallowing will
assist this intrusion (see Fig 6-35).
In either method, TADs can be placed between the
maxillary first molar and second premolar roots or between the first and second molar roots. Molar intrusion
in the mandibular arch is usually more difficult than in
the maxilla. Because microimplant insertion is not recommended on the lingual of the mandibular dental
arch, a lingual bar can be used to control buccolingual
molar inclination. Molar protraction in conjunction with
intrusion causes the mandible to rotate counterclockwise and helps close the bite28 (Fig 6-40).
A microimplant on the zygomatic cortical bone buttress is also recommended for intruding molars more
effectively.33 Even though it is stronger, zygomatic microimplant insertion requires flap surgery, which may
cause soft tissue irritation.
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